Polyphenylene ether is excellent in processability and productivity, and has an advantage of being capable of being efficiently made into products and parts having desired shapes by a molding method such as melt injection or melt extrusion. Polyphenylene ether, making the best use of such an advantage, is broadly used in the field of electric and electronic materials, the field of automobiles, other fields of various types of industrial materials, and the field of food packages, and as materials for parts. Particularly, since polyphenylene ether is excellent in electric characteristics, it is being developed to applications as electronic materials, particular applications requiring the insulation performance.
In the above-mentioned applications as electronic materials, the requirement for, in addition to electric characteristics, the appearance of products has been raised; therefore, the improvement of color tone of polyphenylene ether itself, and the reduction of contamination of molded products with foreign matters (gelled materials, by-products, metals and the like) are demanded.
As technologies to reduce foreign matters contained in polyphenylene ether, there are proposed, for example, a technology in which polyphenylene ether is melt kneaded and thereafter melt filtered (for example, see Patent document 1), and a technology in which the filtration is carried out in every step from the polymerization to the melt kneading (for example, see Patent document 2). There is also proposed a technology in which in order to suppress clogging of a hot runner with metal foreign matters in injection molding utilizing the hot runner, pellets used in injection molding are sorted using a magnetic sorting machine, and only pellets having little metal contamination are taken out (for example, see Patent document 3).
On the other hand, also in the field using polyphenylene ether, the size-reduction and precision enhancement of parts and the thickness reduction of molded products are progressing. In order to maintain the high insulation performance also in such molded products size-reduced, precision-enhanced and thickness-reduced, the electric characteristics of polyphenylene ether are required to be raised.
From the viewpoint of raising the dielectric breakdown resistance and the like out of electric characteristics, metal foreign matters contained in molded products are required to be more highly reduced. The technical development with respect to catalytic metal residues contained in polyphenylene ether has conventionally been made, and there is proposed, for example, a method of making metals in a polyphenylene ether to be 10 ppm or less (for example, see Patent document 4).